1. Field of the Invention
The present invention relates to a method of producing a wet friction material to be used for a clutch, a brake, or the like in a wet frictional engagement device.
2. Description of the Related Art
FIG. 1 shows an example of the configuration of a clutch which is a wet frictional engagement device. In the drawing, torque is transmitted through pressure engagement between a driving plate 2 fitted to a spline portion 51 of a hub 5 inserted in an input shaft 6 and a driven plate 1 fitted to a spline portion 41 of a retainer 4. The reference numerals 3 and 7 designate a pressure plate and a pressing piston respectively.
Such a frictional engagement device is required to be reduced in size and weight, small in operation shock, and large in torque capacity, in view of the energy and environmental problems and the like.
Further, the frictional engagement device is required to cope with the high energy requirement with the promote of high rotational speed and output of an automobile engine, the requirement being extremely high.
In the conventional frictional engagement device, lubricating oil containing an additive for decreasing the coefficient of friction is apt to be much used in order to reduce an operational shock. Therefore, if the frictional engagement device is reduced in size and weight, the torque capacity of the device becomes low inevitably. Since the pressing force is therefore increased to increase the torque capacity, the separation life of a wet friction material is reduced, a heat spot or thermal deformation is caused in an opposite frictional surface (a driven surface), surface temperature rising is caused with the prolongation of the sliding time due to reduction of the coefficient of friction, and a fade phenomenon is generated with the advance hardening of the friction material due to thermally decomposed materials of thermosetting resin.
Further, in order to make the heat-resisting property of a friction material high in the conventional art, it is necessary to make the voids high. That is, the friction material is made porous and lubricating oil is normally contained in the friction material, so that the oil discharged from the inside with compression of the friction material acts to remove the heat generated at the time of pressure engagement.
In order to provide porosity, if the entanglement between fibrous base materials is weakened to obtain high voids (porosity) in the beginning of production of a friction material, however, the mechanical strength of the friction material is reduced. Therefore, there is a limit in the design of production of a porous friction material. Accordingly, there arises a problem that an action to absorb oil into the friction material at the time of releasing of the friction material and to discharge the oil at the time of pressure engagement of the friction material, that is, a so-called sponge effect, is made high so that the heat-resisting life of the friction material is prolonged, while the mechanical to strength is prevented from lowering.